var
a:array[1..10000]of integer;
n,i,d,s:integer;
begin
read(n);
for i:=1 to n do
if abs(n)<10000 then
if n<=0 then d:=-1
else
d:=1;

S:=((2+(n-1)*d)*n div 2 ));

write(s);
end.



(12,26) Fatal: Syntax error, ";" expected but ")" found